Short-term calcitriol administration improves calcium homeostasis in adults with cystic fibrosis

Osteoporosis is a well-defined health risk in cystic fibrosis (CF) patients due to many factors. Vitamin D insufficiency, despite routine cholecalciferol supplementation in CF patients, may contribute to a relative secondary hyperparathyroidism and possibly deficient bone mineralization. An alternate form of vitamin D, calcitriol, was studied to determine short-term effects on fractional calcium absorption and other calciotropic markers in 10 adult CF subjects and in 10 age-, sex- and body mass index (BMI)-matched controls. Serum fractional absorption of 45Ca was determined after a calcium-containing meal prior to calcitriol intervention. Other measurements included serum parathyroid hormone (PTH), ionized calcium, 25-hydroxyvitamin D (25OHD), 1,25-dihydroxyvitamin D (1,25(OH)2D) and urinary calcium:creatinine and N-telopeptide (NTx) concentrations. Both groups were then given calcitriol (0.5 µg p.o. b.i.d. for 14 days) and restudied following the same protocol. Both groups increased their fractional absorption of 45Ca after calcitriol (p=0.015 CF subjects, p=0.001 controls), although calcitriol tended to be less effective in the CF group compared with the controls (p=0.055). Post-prandial serum PTH concentrations were suppressed compared with baseline in both groups (p=0.03 CF subjects, p=0.006 controls). Urinary NTx concentrations, a marker for bone resorption, decreased significantly in CF subjects after calcitriol (96.0±16.0 vs 63.9±12.7 nmol BCE/mmol Cr, p=0.01) and remained unchanged in the control group. The controls had an increase in serum 1,25(OH)2D concentrations (69.9±4.2 vs 90.7±9.6 pmol/l, p=0.02) while there was no significant change in the CF group. Oral calcitriol administration appears to improve markers of calcium balance in adults with CF by increasing fractional absorption of 45Ca and lowering PTH concentrations, similar to its known effects in healthy subjects, while also suppressing urinary NTx, a marker of bone turnover.